1. Field of the Invention
The present invention relates to a system for monitoring a plant. More particularly, it relates to a plant operation monitoring system in which a corrosion potential being one of parameters indicative of a corrosion environment in a nuclear power plant or the like can be measured by utilizing a theoretical computation, and in which the effect of improving the water quality or chemistry of the plant can be studied using the measured result.
2. Description of the Related Art
In a nuclear power plant, oxygen, hydrogen peroxide, etc., which are the radiolytic products of reactor water, exist in the reactor water. A redox system which is constituted by the oxygen, hydrogen peroxide, etc., exhibits a high redox potential. On the other hand, a metallic structural material such as stainless steel, and hydrogen being the radiolytic product of water exhibit low potentials. These facts lead to the problem that the metallic structural material such as stainless steel is corroded (in general, as a corrosion potential is higher, a metal lies in an intenser oxidizing environment). At present, therefore, damage to structural materials in a nuclear reactor attributed to the above corrosion, stress corrosion cracking, etc. is suppressed by performing hydrogen injection. Herein, the hydrogen injection is a technique wherein hydrogen is injected into the reactor water of the nuclear reactor and is reacted with oxidants (such as oxygen, hydrogen peroxide, and intermediate radicals) contained in the reactor water, thereby lowering the concentrations of the oxidants and mitigating a corrosion environment in the reactor.
Meanwhile, the extent of the propagation of such corrosion has a close relation with the corrosion potential. The stress corrosion cracking, for example, has its threshold value at a corrosion potential of about -200 (mV) with reference to a standard hydrogen electrode (SHE). Accordingly, the measurement of the corrosion potential is indispensable for determining the conditions of the hydrogen injection.
In present-day plants, however, places where corrosion potential sensors can be mounted are limited, and the corrosion potentials cannot be measured at all objective parts to-be-monitored. It is therefore required to theoretically simulate the corrosion potential.
Besides, in order to predict and assess the effect of improvements in the water quality or chemistry of the plant owing to the hydrogen injection or the like, the corrosion potential needs to be analyzed in association with the actual chemical components etc. of the reactor water. A technique for theoretically simulating the corrosion potential is also required for this purpose.
Such potential simulations themselves have heretofore been conducted.
By way of example, Japanese Patent Application Laid-open No. 100087/1993 indicates a logical flow chart for a corrosion potential computation in a nuclear power plant system, and the basic reaction rate equation of a single charge-transfer reaction based on the fundamental charge-transfer reaction rate theorem (hereinbelow, this technique shall be called the "prior-art technique A").
Besides, a method of conjecturing a corrosion potential by a computation is stated in proceedings "Corrosion", 48, 3 (1992), pp. 194-205. The effect of the mitigation of the corrosion environment of a nuclear power plant system is also discussed (hereinbelow, called the "prior-art technique B").
A discussion on a corrosion potential is similarly contained in the proceedings "Corrosion", 48, 1 (1992), pp. 16-28 (hereinbelow, called the "prior-art technique C").
In the proceedings "Corrosion", 49, 1 (1993) pp. 8-16, corrosion potentials under the water quality environment of a nuclear power plant system are discussed, and the computed results of the potentials are also introduced (hereinbelow, called the "prior-art technique D").
Incidentally, a corrosion potential which is indicated by an electrochemical mixed-potentials theorem is obtained fundamentally by solving an equation ia-ic=0 to find a potential at which ia=ic holds. Here, "ic" denotes the reaction rate of a reaction system which accepts an electron, while "ia" denotes the reaction rate of a side which releases an electron (a side which is corroded).
According to the electrochemical mixed-potential theorem, the "corrosion potential" is construed to be a "mixed potential" in the state (dynamic equilibrium state) in which the respective reaction rates of the oxidizing reaction of an electron acceptor and the reducing reaction of an electron donor are equal in a certain redox system in which the corrosion reaction of a metal develops (it is to be noted that, in a case where the metal is relevant to the side of lower potential, the mixed potential of the redox system is usually called the "corrosion potential").
The "mixed potential" is determined by the rate at which the redox reaction of higher redox potential (equilibrium potential) accepts electrons from the redox reaction system of lower redox potential, and the rate at which the redox reaction system of the lower redox potential releases electrons.
The equilibrium potential of the redox reaction in each single system can be theoretically obtained through thermodynamical handling represented by the Nernst equation. In a system of two or more coexistent redox reactions, however, the equilibrium potential of the system can no longer be obtained any longer by simply applying the Nernst equation.
The reaction rate of each redox reaction cannot be found from only an overall reaction equation, but it can be determined for the first time by acquiring information down to the rate determining steps of the elementary reaction processes of each reaction. In simulating the corrosion potential of the actual corrosion reaction, accordingly, it becomes important that the rates of the charge transfer reactions relevant to the corrosion reaction are exactly expressed by formulae. Moreover, it is indispensable that the elementary processes determining the reaction rate, such as the rate determining steps of the actual reaction, are theoretically handled in rate theorem fashion.
Nevertheless, any of the logical flow chart or computational contents of the corrosion potential computations stated in the prior-art techniques is utilized in connection with the charge-transfer reaction rate equation concerning the single charge-transfer reaction. By way of example, the computation stated in the prior-art technique B is not based on charge-transfer reaction rate theory, but it is empirical handling. More specifically, notwithstanding that the simulation pertains to the multiple charge-transfer reaction, it handles the reaction rate equation in the single reaction. Accordingly, this technique B is still problematic in theory for the corrosion potential computation which requires the simulation based on a reaction mechanism. Besides, the prior-art technique C mentions nothing about the corrosion potential simulation which is based on the handling based on the charge-transfer reaction rate theory.
In other words, it is difficult to say that any of the prior-art techniques obtains the charge-transfer reaction rates of oxygen, hydrogen peroxide, hydrogen etc. using the handling based on the charge-transfer reaction mechanism. This point will now be explained in more detail.
The overall reaction of the reducing reaction of oxygen is indicated by Chemical formula 1: EQU O.sub.2 +4H.sup.+ +4e.fwdarw.2H.sub.2 O [Chemical formula 1]
Since it is difficult for this four-electron reaction to proceed in one step, the charge-transfer reaction mechanism thereof is a consecutive one in which hydrogen peroxide intervenes as an intermediate as indicated by Chemical formula 2: ##STR1##
Part of the hydrogen peroxide formed as the intermediate in Chemical formula 2 is decomposed in a bulk solution or at the surface of a material to revert to oxygen in accordance with the reaction of Chemical formula 3: ##STR2##
The oxygen formed by this decomposition accepts electrons from the structural material again, and is reduced to water by Chemical formula 2.
The above reactions cannot be handled independently, but the individual reaction processes thereof relate closely to one another.
In spite of this fact, the prior art handles the reactions under the assumption that the rates of the charge-transfer reactions of oxygen, hydrogen peroxide etc. do not affect one another. That is, the aforementioned chemical formula 2 is divided into the steps of the following chemical formulae 4 and 5 so as to obtain the charge-transfer reaction rates which correspond to the respective concentrations of oxygen and hydrogen peroxide: EQU O.sub.2 +2H.sup.+ +2e.fwdarw.H.sub.2 O.sub.2 [Chemical formula 4] EQU H.sub.2 O.sub.2 +2H.sup.+ +2e.fwdarw.2H.sub.2 O [Chemical formula 5]
It is considered that the reactants of hydrogen peroxide contained in Chemical formulae 4 and 5 will be indiscriminate and will act on both the reactions equally. The chemical formulae 4 and 5 cannot be independent of each other, and the reducing reaction of oxygen needs to be handled as the consecutive charge-transfer reaction mechanism given by Chemical formula 2. More specifically, the chemical formula 2 containing hydrogen peroxide as the intermediate consists in the complicated charge-transfer reaction mechanism of both forward and backward reactions. Herein, part of the hydrogen peroxide being the intermediate is decomposed into oxygen, which participates in the reactions of Chemical formula 2 again. Further, the concentration of the oxygen to be formed by the decomposition of the hydrogen peroxide is affected by both the forward and backward reaction rates of each charge-transfer reaction step of Chemical formula 2. It is accordingly impossible to handle the charge-transfer reactions of oxygen and hydrogen peroxide independently.
As thus far explained, neither the charge-transfer reaction rate equation based on the analysis at the elementary reaction level, nor the information items on the rate determining steps of the charge-transfer reactions are used in any of the potential simulations having hitherto been conducted. It can be said, at least, that any of the potential simulations is studied in accordance with a model which is different from the real phenomenon. In addition, the empirical formula is one mere expedient for elucidating an experimental result as to only a specified occasion and specified conditions. Since the empirical formula does not handle parameters admitted extensively and generally, it is often utterly inapplicable to a different environmental situation.
In the nuclear power plant, there coexist a plurality of sorts of electrochemical reaction systems which involve oxygen, hydrogen peroxide, hydrogen, metallic structural materials such as stainless steel, and so forth. There has not heretofore been any example in which the net charge-transfer reaction rates of the whole reaction system having the plurality of sorts of coexistent electrochemical reaction systems in this manner are analyzed and computed from elementary reaction models.